Charged particle beams consisting of protons of heavier ions are successfully used in cancer therapy to destroy tumours by irradiation. A charged particle therapy system using a cyclotron to generate the charged particle beam is for example described in DE 20 2006 019 307. As described by E. Pedroni et al. (Med. Phys. 22 (1) 1995) charged particle therapy systems inter alia use scanning techniques to scan tumour volumes with a charged particle beam in order to effectively destroy the tumour while avoiding damages in neighbouring healthy tissue regions.
In the field of particle therapy, especially when using scanning techniques, it is necessary to switch on and off the beam very quickly, preferably within microseconds. Furthermore, the beam intensity must be adjusted in a wide range within short time, preferably within milliseconds.
In known charged particle therapy systems where the beam is provided by a cyclotron with a horizontal acceleration plane, the quick on/off switching of the beam and the quick adjusting of the beam intensity is done by use of an active vertical deflector system in the inner center of the cyclotron. Such deflector system usually consists of a vertical deflector with two deflector plates being arranged, with respect to the beam direction, downstream from the ion source in the acceleration plane in the very first turns before the beam is accelerated to high energies. In these known systems, if the vertical deflector is not powered, the beam passes straight through the deflector and through an aligned vertical collimator and proceeds to the further acceleration path. If, in these systems, the deflector is powered, the beam is deflected and partly or totally dumped in the vertical collimator. This means that the system requires a—usually high (some kV)—voltage to switch off the beam. With this design, the known vertical deflector systems are not fail-safe with respect to beam switch off. If the powering with a voltage fails, the beam may not be switched off.